Abstract: An electronic locking system comprises a conventional mechanical lock and a key associated therewith, said key having a built-in code transmitter and receiver, preferably an infrared sensitive photo-detector which cooperates with the code transmitter. The receiver is connected to an electronic control unit which is arranged to register a coded signal emitted from the key and transmitted to the receiver. When the signal is acceptable the electronic control unit is activated. The electronic control unit is provided with a memory element contains no information on any coded signal. After transmission of the coded signal of the associated key the coded signal is stored permanently in the memory element whereafter the electronic control unit will accept only precisely this coded signal in order to be activated.